Interplanetary
space is
littered with rocks tens of meters in diameter or less. When these meteoroids
strike
the Earth's
atmosphere at high relative speeds they leave visible trails
created when the intense heat caused by
friction vaporizes them. These are called meteors ("shooting stars").

Fireballs

Most
meteors are tiny specks of dust and rapidly burn up in the atmosphere. Some
are larger and produce spectacular fireballs that are very bright, and
may explode (in some cases with sound heard on the ground). Meteors are
common; you can usually observe a few per hour on any clear night, but
fireballs are rare. The following three images show some recently observed
fireballs.

Hannover, 1995

Perseid Shower, 1996

Peekskill, 1992

The left image shows the path of a fireball recorded
over
Hannover, Germany. The fireball outshines
Sirius and the
constellation Orion
(Ref).
The middle image shows a fireball recorded in the
1996 Perseid meteor shower by the
Dutch Meteor
Society. The curved streaks are star tracks in the timed exposure. The
pulsed nature of the fireball track is produced by the camera which is being
chopped.
The right image shows a fireball observed for over 20 seconds
from Kentucky to New York in 1992
that led to a meteorite fall in Peekskill, New York. In this image the
fireball is beginning to break up into smaller pieces. Here is a movie of the
Peekskill fireball (972 kB;
Source).

Meteor Showers

Meteors enter our atmosphere on a regular basis. On a normal night
you can typically see a few sporadic meteors
per hour. However, at certain times the rate of
observable meteors is much higher. These periods are called meteor showers.
Further, during meteor showers (which usually last a few days), the majority of
the meteors appear to come from a particular point in the sky, called the
radiant of the shower.
The adjacent figure illustrates the position of
the radiant for the
Leonid Meteor Shower, and here is a time-exposed image showing clearly the
radiant location
for the 1995 Lyrid Shower.

The meteor shower is commonly named after the
constellation in which this radiant is found, and occurs annually during a
well-defined time period. For example, the Perseid meteor shower occurs every
year from about July 25th through August 18th, with a peak on August 12, and has
its radiant in the constellation Perseus. In this shower, the typical maximum
number of meteors that can be seen per hour at its peak
is about 70, which is 10 times the
rate of sporadic meteors.

Here is an
observational determination of the radiant for the Quadrantid shower, and
an
MPEG movie
(127 kB, compressed in time) showing 18 meteors of the
Quandrantid shower. The movie was obtained from
this paper,
where a
more detailed and technical discussion of photographing meteors and meteor
showers may be found.

Meteor Showers and Comets

The annual nature of meteor showers suggests that they are associated with the
Earth encountering an unusually large number of meteoroids on particular parts
of its orbit. As the following figures illustrate, meteor showers result when
the Earth passes through the orbit of periodic comets.

Meteor showers result when the Earth encounters
cometary orbits

Perspective makes the meteors
appear to come from a point in the sky

As comets move about their orbits
they leave a stream of debris because
dust
and rocky material is liberated from the head
as the ices vaporize. If the earth
crosses the cometary orbit, this debris leads to an increased number of meteors
(left figure).
These periods are called meteor showers.
That meteors in
meteor showers appear to radiate from a single point in the sky
is an optical illusion, as illustrated in the
figure on the right.
The meteor stream that produces the shower has meteoroids moving on
essentially parallel trajectories, but because of perspective (parallel lines
appear to meet at infinity) the meteors actually entering Earth's atmosphere on
parallel trajectories seem to radiate from a point, as viewed from a site on
Earth.

Some Meteor Showers

We list a few of the stronger meteor showers. Since the orbits of comets are
very well known, in most cases it is possible to list the comet whose orbital
debris is responsible for the shower.

Some Meteor Showers

Name

Date ofMaximum

Meteors / Hourat Max

Parent

Quadrantids

Jan. 4

110

-

Perseids

Aug. 12

68

Comet 1862 III

Orionids

Oct. 21

30

Comet Halley

Leonids

Nov. 17

10

Comet P/Tempel-Tuttle

Geminids

Dec. 14

58

3200 Phaethon

Here is a more extensive
listing
of meteor showers and their characteristics.

Are the Leonids Coming?

Meteor showers often vary in intensity from year to year, presumably reflecting
details of encountering the orbit of the parent comet. One shower that
normally is rather weak, but can at times be strong is the Leonids. As the
preceding table indicates, on the average it only gives 10 meteors per
hour---not even twice the sporadic rate. However, there is a prediction that
the Leonids will grow stronger for the next two years and in 1998 (or perhaps
1999) will
produce 10,000 or more per hour at the peak (when the parent comet is near
perihelion). According to some estimates the
rate will be as high as 40 meteors per second (!) for a short period; a
veritable meteor storm.

Such predictions often do not pan out, but there is historical precedent for
spectacular outbursts from the Leonids. On November 12-13, 1833,
a "tempest of falling stars" was observed in Boston with a rate
"half that of
flakes of snow in an average snowstorm", while in the 1966 the Leonids produced
"a hail of meteors too numerous to count" according to one observer and 150,000
per hour for a 20 minute period according to another
(Reference). Newspaper accounts of the time reported that many people were
awakened from sleep either by the shouts of neighbors, or the light of the
meteors in the sky.

The image on the right
depicts a wood-cut engraving by
Adolf Vollmy based upon an original painting by the Swiss artist Karl Jauslin,
that is in turn based on a first-person account of the 1833 storm by a
minister, Joseph Harvey Waggoner. According to Waggoner's account,
"... it is not possible to give in a picture
a representation of the stars falling at all points of the compass at once. But
they fell in myriad's to the north,
east, south and west. Any representation on paper must at best be a very
limited idea of the reality"
(Reference). Here is another artist's depiction of the
view from Niagara Falls during the 1833 shower, and
another account of the history of the Leonids.

You are encouraged to follow these predictions of Leonid intensity as
November, 1998, approaches. If they are true, it will be a
spectacular sight---perhaps the greatest meteor display of the last 100 years.
To help you keep track, here is
the Leonid '98 Meteor Outburst
Mission HomePage, a set of
predictions for 1998 Leonid rates, and a
history of
Leonid showers. For good measure, here is the orbit and
present position
of the parent comet,
P/Tempel-Tuttle.

The Birth of Modern Meteor Studies

As an historical note, the 1833 Leonid shower in a certain sense marked the
birth of the modern study of meteors, because attempts to explain the
remarkable outburst eventually produced
the realization that the Leonids are
correlated with the periodic comet P/Tempel-Tuttle. This led to a general
understanding of meteor showers as
resulting from the Earth encountering the orbit
of a periodic comet.

These studies indicate that P/Tempel-Tuttle is in an unusual orbit for a comet,
because it
(1) comes close to the Earth's orbit, but
(2) has a period
sufficiently long that its orbit is only slightly perturbed by the planets.
Comets with short periods that come close to the Earth typically do so only a
time or two before gravitational interactions with the planets (primarily
Jupiter) disturb them into other orbits.
But P/Tempel-Tuttle's orbit has been relatively stable for many centuries
and meteor
storms have been seen for more than 1000 years that we now know are associated
with this comet.